February 2013 Archives

For our take-home exam, one of the questions asked about mosaicism and regulation and the differences between the two. I was particularly interested with each mechanism's relation to cell-to-cell interactions and how these interactions imply how these mechanisms exist in the environment naturally.

Mosaicism is the mechanism in which a cell's fate is predetermined by factors it receives during each cleavage. This principle suggests that the mosaic mechanism would not require cell-to-cell interactions. More interestingly, no truly mosaic embryos are known to exist in the environment. Therefore, there must be some other mechanism that is used in order for cell differentiation and embryo development.

Regulation is the mechanism in which an embryo is able to continue developing normally regardless of any changes or destruction. In order to detect these changes, there must be some form of cell-to-cell interactions.

Although there are some organisms that develop as if there is no cell-to-cell interaction and with a predetermined cell fate, it is evident that this is not the only mechanism for development. However, it is important to note an experiment done by Wilhelm Roux that suggested mosaicism as a basis. While studying fertilized frog embryos, Roux destroyed one of the two cells after the first cleavage. The remaining cell developed into only half of an embryo. Therefore, he concluded that mosaicism must be the basis of development for this organism. With this experiment, it only seemed logical that mosaicism was a useful mechanism.

So, if there are no purely mosaic embryos in the environment, what type of mechanism other than mosaicism takes place within a frog? With Roux's experiment, it seems as though mosaicism is a clear answer for this embryonic formation. However, this is only the result because Roux left the killed cell attached, but the embryo did not recognize the cell as being nonfunctional. If the killed cell had been removed, the embryo would have gone through regulation in which it would have developed into a smaller, but normal embryo.

Therefore, regulation is a key mechanism in development. However, with the information presented in class and my own critical thinking, it was hard to come up with this as the answer. Now that I finished my take-home exam, I decided to look up the real 'best' answer for development and it seems as though regulation is it!

Inigo Montoya: "I do not mean to pry, but you don't by any chance happen to have six fingers on your right hand?"
Man in Black: "Do you always begin conversations this way?"

In The Princess Bride (1987), a character named Inigo searches for a six-fingered man who had killed his father when he was a young boy. Although it is supposed to be comical, this variation is not as rare as one may think. In fact, according to the book, Endless Forms Most Beautiful by Sean B. Carroll, about 5 to 17 births out of 10,000 display this variation, known as polydactyly. Other well-known individuals who were born with polydactyly are Anne Boleyn, wife of King Henry VIII, and Antonio Alfonseca, a pitcher for the Marlins. However, it is important to note that this variation has a range of appearances including an extra flap or piece of skin on the side of the hand, individual bones, or duplication of the nail. This mutation has also been seen throughout many vertebrates including mice and chickens.

How might one obtain this variation? Commonly, polydactyly is either inherited or induced experimentally. In fact, scientists believe that there are similarities in the manner of developing this mutation between chickens and humans. However, most advances within this area were made when studying the fruit fly.

While Carroll terms these mutations as 'monsters' or 'mutants,' these variations have incredibly useful suggestions for the rules and process of development. This information can be used to describe the formation of all bodies and body parts of humans and other animals alike. By inducing experimental changes and observing the physical transformations in an organism, scientists are able to track certain genes and investigate how a change could lead to large (or no) effects in a structure.

For lab this week, we were instructed to document certain stages of the development of chicken embryos. Before I get started, I decided to learn a little more about the development of this organism. I think this acquired knowledge might help me in the process of capturing significant stages and structures when conducting the lab.

Once an egg is fertilized by a sperm, the embryo will begin to develop after about five hours, forming the zygote. It will then begin cell division and eventually change into the blastoderm. This stage is comprised of eight cells which will develop into 256 cells after just four hours. The embryo, then, differentiates two layers known as the endoderm and ectoderm. The endoderm will eventually turn into the respiratory and digestive systems, as well as secretory organs. the ectoderm will produce the nervous system, sections of the eyes, the feathers, beak, skin, and claws. Eventually, a third layer will develop known as the mesoderm. This layer is important for producing the skeleton, muscles, circulatory system, reproductive organs, and excretory system.

At day one, the embryonic tissue begins to appear; day three, the heart beats; day eight, feather tracks are seen; day thirteen, the body is slightly covered in feathers; and day twenty, the embryo becomes a chick. Before a human embryo even begins to have a heart beat, the chick embryo is fully developed and ready to hatch.

Below is an image of a chick embryo at 96 hours. As you can see, many of the organs of the chick are distinguishable.